DigitaltoAnalog Converter

1
Digital-to-Analog Converter
2
Simple DAC
MSB
b3
R
2R
b2
-
4R
V0
b1
8R
b0
LSB
Where b3, b2, b1,and b0 are digital inputs.
MSB Most significant bit LSB Least significant
bit
3
Problems With Simple DAC

• Requires too many different resistors for larger
  number of bits.
• Very large resistance are needed for lower
  significant bits, creates extremely small input
  current to op amp.
• The input voltage sources have to be matched with
  the same high and low values.

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R/2R Ladder Network
V3
2R
R
V2
2R
R
V1
2R
R
V0
2R
2R
V12V0, V24V0, V38V0
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DAC With R/2R Ladder
2R
V3
R
R
2R
V2
R
-
2R
V1
Vanalog
R
2R
V0
2R
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Pros Cons of R/2R Ladder DAC

• Pros
• All branches are controlled by a common current
  source.
• Only two resistance values are needed for any
  number of bits.
• Cons
• Small input currents to op amp.

7
DAC Practical Limits
Vout
Offset
Input Word
001 010 011
Vout
Vout
Gain error
Input Word
Input Word
001 010 011
001 010 011
Actual
Ideal
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Integrated Circuit DAC

• National Semiconductor DAC0830/DAC0832 data sheet
  available at this website
• http//www.national.com/
• Some technical terms
• Multiplying DAC output signal input word
  reference voltage
• Settling time the time needed for the output to
  stabilize when an input word changes.

9
Application Example 1 Audio Compact Disc
CD
Laser
DAC
Amplifier
Opto-electro Converter
Typical CD has two 16 bits channels 44.1 kHz
sampling rate 70 minutes music What is a CDs
data storage capacity?
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Application Example 2Automated Test Equipment
Automated Instrument Control
Voltmeter
Voltage Controlled Signal Generator
Circuits (H(?) vs. ?)
Measurement data
Analog voltage
Computer
ADC
DAC
01 05 0A F5 ....
01 01 03 06 ....
Output Measurements
Input control
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Reading and Homework

• Todays coverage Handouts 2
• Next Lecture handouts 3
• Homework 9 due Friday, 2/25
• A typical digital audio tape recorder is a 16 bit
  system with a sampling rate of 48 kHz. How many
  bytes of storage would it take to record a 40
  minute long piece of music?
• If the digital stream of the music data in the
  problem 1 is transmitted to a digital signal
  processing chip in real time, what is the width
  of each individual pulse?
• A 6-bit video DAC produces a maximum output swing
  of approximately 1.25V (analog output voltage
  between -0.625 and 0.625). Determine the output
  voltage for the following digital input words
  000001, 100000, 111111, 011101
• An 8-bit DAC produces an output of 16 mV with an
  input of 00000100. Determine the step size and
  the maximum output signal.